The invention relates to liquid crystal-based light valve systems such as those used in video displays and in particular relates to contrast control in such light valve systems.
A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display device at are small enough to be integrated into a helmet or a pair of glasses so that they an be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists replacement for the conventional cathode-ray tube (CRT) used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display Crevices that incorporate a light valve system that uses as its light control element one or more spatial light modulators based on liquid crystal material.
Liquid crystal-based spatial light modulators are available in either a transmissive form or in a reflective form. The transmissive spatial light modulator is composed of a layer of a liquid crystal material sandwiched between two transparent electrodes. The liquid crystal material is preferably ferroelectric type. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the transmissive spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the transmissive spatial light modulator rotates the direction of polarization of light falling on the pixel. The transmissive spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90xc2x0 so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
Reflective spatial light modulators are similar in construction to transmissive spatial light modulators, but use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, reflective spatial light modulators can be built on a silicon substrate that also accommodates the signal processing electronics that derive the drive signals for the pixel electrodes from the input video signal. A reflective spatial light modulator has the advantage that its pixel electrode drive circuits do not partially occlude the light modulated by the pixel. This enables a reflective spatial light modulator to have a greater light throughput than a similar-sized transmissive spatial light modulator and allows larger and more sophisticated drive circuits as well as the signal processing electronics to be incorporated.
As with the transmissive spatial light modulators, the direction of an electric field (in this case between the transparent electrode and the reflective electrode) determines whether or not the corresponding pixel of the reflective spatial light modulator rotates through 90xc2x0 the direction of polarization of the light falling on (and reflected by) by the pixel. Thus, the polarized light reflected by the pixels of the reflective spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
The resulting optical characteristics of each pixel of both the transmissive and reflective spatial light modulators are substantially binary: each pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field. Polarization analyzers are less than 100 percent efficient, however, absorbing a fraction of the light that should be transmitted in the 1 state and transmitting a fraction of the light that should be absorbed in the 0 state. The ratio of the intensity of light transmitted in the 1 state to the intensity of light transmitted in the 0 state is known as the xe2x80x9ccontrast ratioxe2x80x9d or simply as xe2x80x9ccontrast.xe2x80x9d A contrast ratio of at least 100:1 is required for excellent image quality and is usually found in CRT based displays.
To produce the grayscale required for conventional display devices, the apparent brightness of each pixel is varied. In nematic liquid crystal-based spatial light modulators, grayscale is achieved by changing the voltage of the drive pulse. Ferroelectric liquid crystal-based spatial light modulators, however, are digital devices switching between the 1 state and the 0 state almost independent of drive voltage. Grayscale in ferroelectric liquid crystal-based spatial light modulators is therefore achieved by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
To produce color output required for conventional display devices, a single spatial light modulator may be used or multiple spatial light modulators may be used. In order to produce a color output from a single spatial light modulator, the spatial light modulator is illuminated sequentially with light of different colors, typically red, blue, and green. This sequential illumination may be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a xe2x80x9cwhitexe2x80x9d light source with sequential color filtering. For purposes of this description a xe2x80x9cwhitexe2x80x9d light source is one that emits light over a broad portion of the visible light spectrum. In either case, each of the sequential colors is modulated individually by the spatial light modulator to produce three sequential single-color images. If the sequence of single-color images occurs quickly enough, a viewer of the sequential single-color images will be unable to distinguish the sequential single-color images from a full-color image.
To produce color output using multiple spatial light modulators, each of the spatial light modulators is simultaneously illuminated with a different colored light. This can be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a xe2x80x9cwhitexe2x80x9d light source with a color separator. Typically three spatial light modulators are used, one illuminated with red light, one with blue light, and one with green light. Each of the spatial light modulators modulates the colored light that illuminates it to form a single-colored image, and the single-colored images from each of the spatial light modulators are combined into a single full-color image.
FIG. 1 shows part of a conventional display device 5 incorporating a conventional reflective light valve 10 that includes the reflective ferroelectric liquid crystal-based spatial light modulator 12. Other principal components of the light valve are the polarizer 14, the beam splitter 16 and the analyzer 18. The light valve is illuminated with light from the light source 20, the light from which is concentrated on the polarizer using a reflector 22 and collector optics 24. The light output by the light valve passes to the imaging optics 26 that focus the light to form an image (not shown). The light valve 10, light source 20 and imaging optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 20 enters the light valve 10 by passing through the polarizer 14. The polarizer polarizes the light output from the light source. Alternatively, a polarized light source (not shown) can be used and the need for the polarizer 14 would be eliminated. The beam splitter 16 then reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator 12. The beam splitter can additionally or alternatively be a polarizing beam splitter configured to reflect light having a direction of polarization parallel to the direction of polarization of the polarizer 14 towards the spatial light modulator 12. The spatial light modulator 12 is divided into a two-dimensional array of picture elements (pixels) that define the spatial-, resolution of the light valve 10. Light reflected from the spatial light modulator can pass to the beam splitter 16 which transmits a fraction of the reflected light to the analyzer 18. If the beam splitter is a polarizing beam splitter, however, only light having a direction of polarization orthogonal to the direction of polarization imparted by the polarizer will be transmitted, effectively acting as an analyzer, and the need for an independent analyzer may be eliminated.
The direction of an electric field in each pixel of the spatial light modulator 12 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90xc2x0 relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the beam splitter 16 and the analyzer 18 and is output from the light valve 10 through the imaging optics 26 depending on whether or not its direction of polarization was rotated by the spatial light modulator.
More specifically, the polarizer 14 polarizes the light generated by the light source 20 that passes through the collector optics 24 either directly or after reflecting off reflector 22. The polarization is preferably linear polarization. The beam splitter 16 reflects the polarized light output from the polarizer towards spatial light modulator 12, and the polarized light reflected from the spatial light modulator transmits to the analyzer 18 through the beam splitter 16. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
For purposes of this description, the terms parallel and orthogonal will be used to describe directions of polarization and directions of maximum transmissivity and minimum transmissivity. When these terms are used within this description, it is understood that they relate to the optical characteristics of the light and of the various components that comprise the light valve and not necessarily to their spatial relationships. For example, when polarized light reflects from a mirror at an angle of incidence of 45xc2x0, the polarized light is reflected at an angle of 90xc2x0 relative to the incident light. Even though the incident light and the reflected light are spatially orthogonal to one another, the direction of polarization of the reflected light will be optically unchanged from the direction of polarization of the incident light. Thus, the direction of polarization of the reflected light may be said to be parallel to the direction of polarization of the incident light. In addition, as used herein the term parallel shall include directions that are both parallel and anti-parallel, i.e., having a direction 180xc2x0 opposed, to the original direction.
The spatial light modulator 12 is composed of a transparent electrode 28 deposited on the surface of a transparent cover 30, a reflective electrode 32 located on the surface of the semiconductor substrate 34, and a ferroelectric liquid crystal layer 36 sandwiched between the transparent electrode 28 and the reflective electrode 32. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600xc3x971200 pixel electrodes. An exemplary pixel electrode is shown at 38. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the beam splitter 16.
A drive circuit including signal processing electronics may be located in the semiconductor substrate 34. An example of such a drive circuit 50 with signal processing electronics 52 and an exemplary pixel is shown in FIG. 2. Regarding this figure and those that follow, it is noted that identical reference numerals are used to designate identical or similar elements throughout the several views, and that elements are not necessarily shown to scale. The drive circuit has external connections to a voltage source with voltage level VS, ground, and a video signal 40. The drive circuit applies a drive signal to the pixel electrode 38 of each pixel of the spatial light modulator 12. The drive signal can be switched between two voltage levels, VS and ground. The switching of the drive signal supplied to the pixel electrode 38 is controlled by the signal processing electronics 52 though two transistors T1 and T2 in response to a portion of the video signal 40. The gates of the two transistors are in electrical communication with the signal processing electronics at nodes P+1 and Pxe2x88x921, respectively. Other nodes are provided in the signal processing electronics for providing a drive signal to the remaining pixels (not shown).
Two matched resistors, R1 and R2, connected in series between VS and ground, are used to provide a voltage level equal to xc2xdVS between the resistors. An isolation amplifier A with unity gain has its input connected between the two matched resistors and its output connected to and the transparent electrode 28. The isolation amplifier ensures that the voltage level at the transparent electrode is maintained at xc2xdVS, a fixed voltage level mid-way between the voltage levels of the drive signal. Without the isolation amplifier A, transient currents that occur at the transparent electrode when the drive signal at the pixel electrode 38 switches between VS and ground could affect the amount of the current drawn through one of the matched resistors and thus alter the voltage level between the resistors.
The potential difference between the pixel electrode 38 and the transparent electrode 28 establishes an electric field across the part of the liquid crystal layer 36 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
Referring back to FIG. 1, since light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes, the reflective spatial light modulator 12 is structured as a quarter-wave plate. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 36 is chosen to provide an optical phase shift of 90xc2x0 between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* surface stabilized ferroelectric liquid crystal material having an angle of 22.5xc2x0 between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle of about 45xc2x0. Consequently, if the director is aligned parallel to the direction of maximum transmission of the analyzer 18 with one polarity of the electric field, reversing the direction of the electric field will rotate the direction of polarization of light reflected by the pixel through 90xc2x0. This will align the direction of polarization of the light orthogonal to the direction of maximum transmission of the analyzer, and will change the pixel from its 1 state, in which the pixel appears bright, to its 0 state, in which the pixel appears dark.
Since the direction of maximum transmission of the analyzer 18 is orthogonal to the direction of polarization defined by the polarizer 14, light whose direction of polarization has been rotated through 90xc2x0 by a pixel of the spatial light modulator 12 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the imaging optics 26 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel. When a pixel appears bright, it will be said to be in its 1 state, and when the pixel appears dark, it will be said to be in its 0 state.
The direction of maximum transmission of the analyzer 18 can alternatively be arranged parallel to that of the polarizer 14, and a non-polarizing beam splitter can be used as the beam splitter 16. In this case, the spatial light modulator 12 operates in the opposite sense to that just described.
In a miniature, wearable display, the imaging optics 26 are composed of an eyepiece that receives the light reflected by the reflective electrode 32 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the imaging optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit (not shown) for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 40 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 3A-3E illustrate the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1 during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 40. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 3A.
FIG. 3B shows the drive signal applied to the exemplary pixel electrode 38. The transparent electrode 28 is held at a voltage level of VS/2, so that changing the voltage level on the pixel electrode from 0 (ground) to VS reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36. The level of the drive signal is VS for a first temporal portion 1TP of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first temporal portion 1TP of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is VS during the first temporal portion of the illumination period. Finally, the level of the drive signal changes to VS for the second temporal portion 2TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the drive signal is 0 and VS for times equal to the times that it was at VS and 0, respectively, during the illumination period. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 40.
FIG. 3C shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle xcex1 between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle xcex1 close to zero and absorbs light having an angle xcex1 close to 90xc2x0. In each display period, the angle xcex1 has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 3D shows the condition of a fast-acting light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period. Alternatively, the light source could remain on and a shutter (not shown) could be used to control whether the light generated by the light source illuminates the spatial light modulator 12. For example, an open shutter would correspond to the light source being ON and a closed shutter would correspond to the light source being OFF.
FIG. 3E shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 20 is OFF during the balance period.
The light valve 10 shown in FIG. 1 can also be adapted to provide a colored light output to the imaging optics 26. One way that this can be done is by replacing the xe2x80x9cwhitexe2x80x9d light source 20 with three colored light sources such as a red, blue and green LEDs (not shown), each illuminating the spatial light modulator 12 sequentially. This would require a balance period after each sequential illumination period. Another way that a colored light output can be provided is by replacing the single reflective spatial light modulator 12 shown in FIG. 1 with three reflective spatial light modulators and a color separator for separating the light into three component colors.
An example of one such color configuration is depicted in FIG. 4. In FIG. 4, the color separator is a series of three dichroic plates 42, 43, 44, each having an associated reflective spatial light modulator 12. Each of the dichroic plates is configured to reflect light in a band of wavelengths (colorband) particular to that dichroic plate and to pass the remaining wavelengths of light. Thus, if the light source 20 is a xe2x80x9cwhitexe2x80x9d light, emitting visible light across the entire visible color spectrum, a particular portion of the color spectrum may be reflected by each dichroic plate its associated reflective spatial light modulator simultaneously. This eliminates the need for sequential illumination and improves the perceived brightness of the color pixels passing through the analyzer.
For example, the dichroic plate 42 nearest the beam splitter 16 might reflect red-colored light toward its associated spatial light modulator 12 while the center dichroic plate 43 reflects green-colored light toward its associated spatial light modulator and the dichroic plate remote from the beam splitter 44 reflects blue-colored light towards its spatial light modulator. When the light source 20 is ON, as shown if FIG. 2, the colored light reflected by the dichroic plates passes to each of the three reflective spatial light modulators 12. Each of the three reflective spatial light modulators is capable of reflecting pixels of the colored light back at its associated dichroic plate in a manner consistent with the above description of the operation of the spatial light modulator shown in FIG. 1.
The majority of the colored light reflected by each of the spatial light modulators 12 will be reflected by its associated dichroic plate toward the analyzer 18 since the light reflected by each spatial light modulator will retain the characteristic wavelengths of light originally reflected by its respective dichroic plate. When the combined colored light from each of the three reflective spatial light modulators 12 passes through the analyzer, a full color image can be formed by the imaging optics 26.
FIG. 5 depicts the use of a color separation cube 46, sometimes known as an x-cube or crossed-dichroic cube, as a color separator in place of the three dichroic plates. As with the three dichroic plates, the color separation cube separates three distinct color bands from the xe2x80x9cwhitexe2x80x9d light created by light source 20 and directs each of the color bands to a particular spatial light modulator 12. The color separation cube 46 also recombines the light reflected from each of the spatial light modulators 12 and directs the combined light toward the analyzer 18. The use of a color separation cube allows for a more compact design utilizing three spatial light modulators than can be achieved using three separate dichroic plates.
FIG. 6 depicts the use of a third type of color separator, a three-prism color separator 48 (sometimes known as a Philips cube or Philips prism), in a light valve utilizing three spatial light modulators to generate a color image. The design and use of a three-prism color separator is described in detail in U.S. Pat. No. 5,644,432, the contents of which are incorporated herein by reference. Like the previously described color separators, the three-prism color separator separates three distinct color bands from the xe2x80x9cwhitexe2x80x9d light created by light source 20 and directs each of the color bands to a particular spatial light modulator 12. The three-prism color separator 48 also recombines the light reflected from each of the spatial light modulators 12 and directs the combined light toward the analyzer 18.
Whether the light valve includes a single spatial light modulator 12 or three, it is important that a high contrast ratio between the 1 state and the 0 state be maintained. The precise alignment of the direction of polarization of light reflected from each pixel either parallel to or orthogonal to the direction of maximum transmissivity of the analyzer 18 is of critical importance in order to maintain the high contrast ratio. Even slight misalignment will cause the a portion of the light which should be transmitted by the analyzer 18 in the 1 state to be absorbed and a portion of the light which should be absorbed by the analyzer in the 0 state to be transmitted. Light transmitted through the analyzer in the 0 state is known as xe2x80x9cdark state luminancexe2x80x9d and significantly influences image quality and quickly drives the contrast ratio down.
The precise alignment of the direction of polarization reflected by the pixels is affected by a number of mechanical tolerances encountered during the assembly of the light valve 10. Tolerances in the mechanical alignment of the polarizer 14, the spatial light modulator 12 and the analyzer 18 are typically plus or minus one-half degree and contribute to misalignment. In addition, during assembly of the spatial light modulator 12, the angle at which the normal to the smectic layers of the ferroelectric liquid crystal material 36 is aligned relative to the substrate 34 (pre-alignment angle) has a tolerance that is also typically plus or minus one-half degree that may contribute to misalignment. Once the light valve 10 is assembled, these tolerances result in a permanent fixed offset.
In addition to the permanent fixed offset, however, the direction of polarization of light reflected from the pixels is affected by variations in the tilt angle of the ferroelectric liquid crystal material with temperature change. Tilt angle is the angle that the director of the ferroelectric liquid crystal material switches through when the direction of the electric field across the ferroelectric liquid crystal material is reversed. The tilt angle is symmetric around the pre-alignment angle. Thus, if the director of the liquid crystal material at a particular temperature forms an angle of 22.5xc2x0 with the normal to its smectic layers when exposed to an electric field having a forward direction, the director switches through a tilt angle of 45xc2x0 when the direction of the electric field reverses. Tilt angle usually changes by between one and two degrees during a change from room temperature to operating temperature. Operating temperature is typically 60xc2x0 C. with the increase in temperature coming from heat generated by the operation of the drive circuits 50 and the absorption of energy from the light illuminating the spatial light modulator.
FIGS. 7A and 7B show how variations in tilt angle with temperature affect operation of the light valve. FIG. 7A depicts an analyzer 18 and a ferroelectric liquid-crystal-based spatial light modulator 12 at room temperature, such as when a light valve is first turned on. FIG. 7B shows the same analyzer and spatial light modulator at normal operating temperature. The direction of maximum transmissivity of the analyzer is indicated in both figures by line 54 with closed arrow heads, while the orthogonal direction of minimum transmissivity is shown by the line 56 with open arrow heads. The direction of polarization of light reflected by pixels having an electric field in the forward direction is depicted in both figures by line 58 with closed arrow heads. The direction of polarization of light reflected by pixels having an electric field in the reverse direction is depicted in both figures by line 60 with closed arrow heads.
In FIG. 7A, the angle between the direction of polarization of the light reflected from pixels having a forward electric field and pixels having a reverse electric field is indicated as a room temperature angle (RTA) greater than 90xc2x0. While RTA would actually be only slightly greater than 90xc2x0, its magnitude has been exaggerated in the figure for clarity. The slight misalignment between the direction of polarization of light reflected from the spatial light modulator 58, 60 and the direction of maximum or minimum transmnissivity of the analyzer 54, 56 reduce the contrast ratio of the light valve. The misalignment between the direction of polarization of light having a direction 60 and the direction of minimum transmissivity 56 has a substantially greater effect on the contrast ratio than does the misalignment between the direction of polarization of light having a direction 58 and the direction of maximum transmissivity 54.
In FIG. 7B, the angle between the direction of polarization of the light reflected from pixels having a forward electric field and pixels having a reverse electric field is indicated as an operating temperature angle (OTA) less than 90xc2x0. While OTA would actually be only slightly less than 90xc2x0, its magnitude has been exaggerated in the figure for clarity. Again, the slight misalignment between the direction of polarization of light reflected from the spatial light modulator 58, 60 and the direction of maximum or minimum transmissivity of the analyzer 54, 56 reduce the contrast ratio of the light valve. In this case, the performance of the light valve at neither room temperature or operating temperature is optimal, but a compromise exists so the performance at room temperature is not noticeably worse than it is at room temperature.
It is possible for manufacturers to minimize the permanent fixed offset by actively aligning the components. This process requires that a skilled assembler illuminate the system with a light source 20 and mechanically align all the components once they have reached operating temperature until the contrast ratio is maximized. The process of active alignment is both time consuming and expensive and thus not well suited for mass production. In addition, light valves can be designed with heaters to keep the spatial light modulator at operating temperature even when not in use so that the variation in tilt angle with changes in temperature will not affect performance of the system. The heated light valve systems minimize or eliminate warm up time and provides stability in the contrast ratio. The heated light valve systems, however, requires the expense of the heater components and have the disadvantage that they consume power even in the xe2x80x9coffxe2x80x9d state and will not work if the system is turned off by disconnecting the light valve (and thus the heaters) from a power source.
Factory settings and heaters, however, do not offer the dynamic minute to minute adjustment of contrast control that may be appropriate, especially during warmup, nor do they offer a user the ability to adjust contrast. Consequently, what is needed is a light valve system with automatic contrast correction and the ability for a user to change contrast settings.
The invention provides a ferroelectric liquid crystal-based light valve system with contrast control and a method for controlling contrast in a ferroelectric liquid crystal-based light valve system. The light valve system with contrast control includes a light input, a ferroelectric liquid crystal-based spatial light modulator, an analyzer, and a voltage controller. Light having a first direction of polarization is received through the light input. The ferroelectric liquid crystal-based spatial light modulator includes signal processing electronics and an array of pixels. The array of pixels is configured to receive light from the light input and includes an array of pixel electrodes, a transparent electrode, and a layer of ferroelectric liquid crystal material.
Each pixel electrode in the array of pixel electrodes defines the location of one pixel in the array of pixels and is independently switchable between a first voltage level and a second voltage level by the signal processing electronics. One of the first voltage level and the second voltage level may be ground and the other may be a variable input voltage level. The first voltage level corresponds to a first pixel condition in which light exiting the pixel has a second direction of polarization. The second voltage level corresponds to a second pixel condition in which light exiting the pixel has a third direction of polarization that is within about 5 degrees of orthogonal to the second direction of polarization. The signal processing electronics may have an input voltage level equal to one of the first voltage level and the second voltage level.
The transparent electrode is held at a third voltage level between the first voltage level and the second voltage level. The third voltage level may be equal to half the difference between the first voltage level and the second voltage level. The layer of ferroelectric liquid crystal material is sandwiched between the array of pixel electrodes and the transparent electrode.
The analyzer has orthogonal directions of maximum transmissivity and minimum transmissivity and is also positioned to receive the light exiting the array of pixels. The analyzer is additionally aligned with the direction of minimum transmissivity within about 5 degrees of the second direction of polarization, and the direction of maximum transmissivity aligned within about 5 degrees of the third direction of polarization.
The voltage controller is configured to adjust at least one of the first voltage level and the second voltage level through a first range of voltages and a second range of voltages, respectively, and may be configured not affect the input voltage level of the signal processing electronics. As the voltage controller adjust the variable input voltage level though its range of voltages, at least one of the second and the third direction of polarization may rotate by at least 2 degrees. The voltage controller may operated in response to a user input.
The light valve system may further comprise a light intensity sensor and a feedback circuit in electrical communication with the light intensity sensor and the voltage controller. The light intensity sensor is configured to detect the intensity of the light having the second direction of polarization transmitted through the analyzer. The feedback circuit may be configured to automatically minimize the light having the second direction of polarization illuminating the light intensity sensor.
The method of controlling contrast in a ferroelectric light valve system begins by providing a ferroelectric liquid crystal-based spatial light modulator that includes a plurality of pixels and signal processing electronics. The signal processing electronics independently switch an electric field across each of the plurality of pixels between a forward direction and a reverse direction.
Next, an analyzer is provided that has a direction of minimum transmissivity. The analyzer is mechanically aligned with the ferroelectric spatial light modulator. The alignment is such that when the plurality of pixels is illuminated by light having a first direction of polarization, the light exiting those of the pixels having an electric field with a direction that is one of either forward or reverse has a second direction of polarization that is aligned within 5 degrees of the direction of minimum transmissivity.
The plurality of pixels are then illuminated with light having the first direction of polarization and a video signal is provided to the signal processing electronics. The electric field across a majority of the plurality of pixels is then switched independently between the forward direction and the reverse direction in response to the video signal.
Next, a contrast control signal is detected. Detecting the control signal may include checking for an input from a contrast control user interface. Detecting the control signal may also include providing an automatic contrast control feedback circuit. If provided, the feedback circuit may include a light intensity sensor configured to detect the intensity of light exiting at least one of the plurality of pixels and transmitted through the analyzer. Detecting the control signal may also include switching the at least one of the plurality of pixels to one of the forward direction and the reverse direction electric field so the direction of polarization of light exiting the at least one of the plurality of pixels is aligned within 5 degrees of the direction of minimum transmissivity. Alternatively, the majority of the pixels may be uniformly switched to one of the forward electric field and the reverse electric field and the intensity of light detected with the light intensity detector.
The second direction of polarization is then rotated by changing the magnitude of the electric field across the plurality of pixels in response to the control signal. The direction of polarization of the light exiting the plurality of pixels may be rotated until the intensity of light received by the light intensity sensor is minimized. Alternatively, the direction of polarization of the light exiting the plurality of pixels can be rotated until the direction of polarization of the light exiting the plurality of pixels having one of a forward direction electric field and a reverse direction electric field is parallel to the direction of minimum transmissivity.
Accordingly, the invention provides a light valve system with contrast control and the method of controlling contrast in a light valve system. Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating, by way of example, the principles of the invention.